The present invention relates to a magneto-optical reproducing optical pickup for reproducing an information signal recorded on a recording medium, such as a magneto-optical disc or the like and, more particularly, to a magneto-optical reproducing optical pickup using an optical waveguide.
FIG. 1 of the accompanying drawings shows an example of a magneto-optical reproducing pickup using individual parts. As shown in FIG. 1, straight line polarized light (simply referred hereinafter as "TE wave") having a predetermined polarizing plane emitted from a light source 101 composed of a laser light source formed of a laser diode and a polarizing plate is radiated through a partial beam splitter 102 (referred to hereinafter as "partial BS") on a recording medium, i.e., magneto-optical recording medium 103. Light reflected from the magneto-optical recording medium 103 is reflected by the partial BS102 to an analyzer (analyzed by 45degrees) 104 which in turn introduces that reflected light into a pair of photodetecting devices 105a, 105b. Thereafter, the reflected light is introduced into a differential amplifier 106 of the following stage as an electrical signal (photodetected output). The pair of photodetecting devices 105a, 105b and the differential amplifier 106 constitute a differential detecting circuit 107. In this case, the partial BS102 has a characteristic such that it can reflect 20% of TE wave, for example, and reflect substantially 100% of TM wave of a polarized plane perpendicular to the TE wave.
Most (80%) of the TE wave from the light source unit 101 is radiated on the magneto-optical recording medium 103. Although the reflected light whose polarizing plane is rotated by an optical-magnetic mutual action (Kerr effect) in response to a magnetization of a recorded signal on the magneto-optical recording medium 103 is again introduced into the partial BS102, almost all of the TE wave component generated by rotation (kerr rotation) based on Kerr effect is reflected.
The TE wave component reflected by the partial BS102 is polarized and analyzed by the next analyzer 104 at 45 degrees and introduced into the pair of photodetecting devices 105a, 105b. In this case, an amount of light incident on one photodetecting device 105a, for example, is decreased in response to recorded information on the magneto-optical recording medium 103, i.e., the magnitude of the Kerr rotation so that it is possible to generate a reproduced signal by differentially detecting the detected outputs of the two photodetecting devices 105a, 105b by the differential detecting circuit 107.
As a magneto-optical reproducing pickup which can be miniaturized, reduced in weight and satisfactorily mass-produced, there is known a magneto-optical reproducing pickup which uses an integrated optical technology utilizing an optical waveguide, i.e., magneto-optical reproducing pickup in which light emitted from an optical waveguide is directly radiated onto a magneto-optical recording medium and rotation of a polarizing plane of light passed through or reflected onto the magneto-optical recording medium (see Japanese laid-open patent publications Nos. 60-224139 and 1-279432).
As shown in FIG. 2, light L propagating through an optical waveguide 111 becomes elliptical polarized light because propagation constants between corresponding modes in the polarized directions perpendicular to each other are different.
When polarized light incident on the surface of a magneto-optical material becomes elliptical, information of rotation of a detected polarized plane becomes small. If the direction of polarized light incident on the optical waveguide is in agreement with one stationary mode, light can pass through one optical waveguide when such light is introduced under the condition that its polarized state is maintained. However, if reflected light from the surface of the magneto-optical material is again introduced into the optical waveguide, when such light is returned, another stationary mode component also is generated by the rotation of the polarized plane by the magneto-optical material. As a result, a phase difference between the two modes makes emitted light become elliptical polarized light.
Differential detection using polarizers whose polarizing directions are respectively inclined at .+-.45 degrees relative to the polarized direction of incident light will be described with reference to FIGS. 3 and 4. In the schematic diagrams shown in FIGS. 3 and 4, a vertical axis represents a TE mode polarizing wave direction and a horizontal axis represents a TM mode polarized wave direction. Then, FIGS. 3 and 4 show polarized directions (vectors) of two reflected lights, i.e., reflected light 121 in the upward magnetization and reflected light 122 in the downward direction, respectively.
In FIGS. 3 and 4, lines m and n extending in the directions of .+-.45.degree. relative to the horizontal axis represent polarizing directions of two polarizers whose polarizing directions are inclined at +45.degree. and -45.degree. relative to the polarized direction of incident light 123. In this example, let it be assumed that light of TE mode is introduced into the magneto-optical recording medium.
Specifically, when a magnetization direction of the magneto-optical recording medium is inverted, the polarized directions of the two reflected lights 121 and 122 are rotated by Kerr effect by the +.theta. or -.theta. relative to the horizontal axis TM. At that time, the differential detector outputs a difference (line AA'--line aa') or (line BB'--line bb') at peak-to-peak.
When these reflected lights 121, 122 propagate through optical wave guides and become elliptical polarized lights 124, 125 as shown in FIG. 4, a difference (=line AA'--line aa') between line AA'and line aa' connecting tops of long axes of respective ellipses and a difference (=line BB'--line bb') between line BB' and line bb' connecting tops of long axes of respective ellipses are reduced as compared with a difference (=line AA'--line aa') between line AA' and line aa' and a difference (=line BB'--line bb') between line BB' and line bb' shown in FIG. 3.
.theta. assumes a rotational angle of the polarizing plane in the magneto-optical material and .psi. assumes a phase difference between two modes generated when light passes one of the optical wave guides. Then, when a differential detection is carried out on the polarized planes of the direction inclined .+-.45.degree. from the incident light polarized directions, a detected signal is proportional to cos.psi.sin2.theta..
Thus, if the phase difference .psi. is selected to be a positive integral of .pi., then no signal is deteriorated. In this case, the phase difference .psi., is in proportion to a waveguide length and there is a method of adjusting the phase difference on the basis of the waveguide length. According to this method, the waveguide must be manufactured with accuracy of wavelength order, which is not practical. Further, there is the problem that this method cannot cope with the change of optical parameters of waveguide due to the change of temperature and the change of polarized state of emitted light due to fluctuation of wavelength of light source.